The compounds MIBG (Iobengane, meta-iodobenzylguanidine or mIBG), and the astatine analog MABG (meta-astatinobenzylguanidine), are used diagnostically to detect a variety of disorders. The compounds comprise a radioactive iodine or astatine atom, and a protonated guanidine (i.e., guanidinium) moiety. Available synthesis methods for the preparation of MIBG and MABG require protection of the guanidine moiety during the synthesis steps used to attach the radioactive iodine or astatine atom. Subsequent deprotection of the guanidine moiety results in the formation of radioactive byproducts, impurities and solvent waste. Moreover, radioactive halogen atoms have short half-lives, and deprotection and purification procedures consume much of the limited time within which the compounds are useful. There remains a need for intermediates and methods for the efficient synthesis of MIBG, MABG, and related compounds, said intermediates and methods avoiding the use of guanidine protecting groups.